If a horse starts to overheat, it’s essential you act, and fast. Gil Riley MRCVS explains how we can spot and react to hyperthermia in horses.
Overheating, also known as hyperthermia or heat stress, is a condition that is most usually seen in horses that are involved in strenuous exercise on hot, humid days.
While more commonly seen in horses living in warmer climates, it can be a possible reason for veterinary care on a hot day in the UK, and is perhaps destined to become more common with global warming.
Overheating is also seen in horses that have moved from a cold climate to a warm one to compete. It’s even possible for horses to overheat when not exercising on particularly warm or humid days, if they are over-rugged, unclipped, carrying excessive weight or the horse is rendered less resistant to heat by the presence of another problem, such as Cushing’s.
Overheating is most frequently seen when horses are performing in disciplines such as endurance, racing or eventing. A horse performing strenuous exercise is often pushed to the limits of his recovery mechanisms and, while in most cases these mechanisms allow the horse to finish the exercise with no problems, sometimes his ability to recover is inadequate, putting him into a shock-like state.
With shock, one or more organs, such as the muscles, kidneys, central nervous system, or clotting system, will stop functioning properly. This can result in death.
Therefore a horse with hyperthermia should always be considered an emergency.
Signs of overheating
Typically, a horse will overheat during exercise and signs will become immediately apparent when the exercise finishes.
The first sign something is amiss is that he appears distressed and anxious. He will have a high heart and respiratory rate that does not decrease with rest, and his skin will feel hot.
He may show signs of circulatory shock such as pale, dry mucous membranes (sticky gums), increased capillary refill time (gums that take longer than one second to turn pink again from white after they have been pressed with a fingertip), a weak irregular pulse, and no gut sounds, or signs of colic.
He may appear wobbly and demented, unstable on his legs, even falling to the ground before rising and falling again – particularly upsetting for the horse and for anyone witnessing it.
While failure to treat promptly can result in death, those horses that survive the overheating may go on to develop other, often life-threatening, conditions such as laminitis, kidney failure or diarrhoea.
Thermoregulation system
Overheating occurs when there is a failure in the horse’s thermoregulation system, which is the “thermostat” that self-adjusts the internal body temperature.
The goal of the thermoregulatory system is survival by keeping the internal organs functioning properly at the ideal equine body temperature (around 37.5°C). The environmental temperature can affect body temperature significantly; when it’s cold, the equine body works to conserve and increase internal heat, while on hot days it dissipates heat and reduces body temperature through the skin, its interface with the air.
Thermal receptors in both the internal organs and in the skin send messages to the hypothalamus, a small part of the brain responsible for many hormonal functions, which then activates various thermoregulatory mechanisms to trap or release heat as necessary. The horse converts energy into movement with about 25% efficiency, with much of the remainder generating heat.
During exercise, there is a significant increase in the amount of heat produced, with the rate of heat production proportional to how hard the muscles work – the faster a horse goes, the more heat it produces.
This heat isn’t all wasted, as some is used to warm the body when it’s cold outside, and some is used to keep the muscles warm while working. What is left can, in cool-warm conditions, be safely dissipated.
It is following intense or long exercise sessions on hot, humid days that the mechanisms of thermoregulation struggle to dissipate the excess heat and when overheating may occur.
The skin and vasodilation
The single most important means the horse has for getting rid of the enormous heat load generated during exercise is via the skin’s vasodilation system.
As the horse’s internal temperature increases, the blood vessels in the skin dilate, routing as much as a third of the horse’s blood at any time to the skin, permitting the heat to escape.
This system comes under threat when too much blood gets pumped away from the internal organs which can then suffer oxygen deficiency, the basis of the damage caused by overheating.
Body temperatures of up to 38.5°C can be considered normal, but beyond this the horse will start to experience overheating. It is at around 41°C when vasodilation directs excessive blood away from the organs to the skin, a major emergency that can result in death.
Above 42.5°C, coagulation can occur in proteins in the brain.
Sweating: How effective is it?
Humans and horses are the only two mammals that use sweating for thermoregulation, but it is a less effective tool in horses than in humans. This is because not only do humans have much more skin surface area for their body size, but the hugely greater muscle mass of the horse means it produces much more heat than we do.
When horses sweat, the fluid evaporates off their skin into the air, pulling with it body heat, resulting in a cooling effect. Horses have a huge number of sweat glands, and they can produce as much as 20 litres of sweat in an hour.
Surprisingly, this excessive production doesn’t help cool the horse that much, with only about 25–30% of it evaporating; the rest sitting on the body and running down the legs. On humid days, the air is already saturated with water, so evaporation is further reduced.
It should also be remembered that sweat is not just water, but also contains various salts. These salts, when broken down into their chemical components, are referred to as electrolytes.
When performing fluid replacement in horses that have overheated, electrolyte replacement should be prioritised.
A breath of fresh air
The lungs account for approximately 25% of the heat lost by a horse.
The capacity of the respiratory tract to dissipate heat from the body becomes more important under conditions of high humidity when evaporative conditions are not favourable. In such cases, the horse’s rate of breathing will greatly increase, from around eight breaths a minute to more than 100.
Fast, effective treatment
Affected horses must be treated immediately. Provided that intervention is timely, most horses will make a complete recovery.
Call the vet and, while waiting for them to arrive, if possible, move the horse slowly to a cool, shady area. Next, efforts should be directed to cooling the core temperature of the horse.
Remember, stay safe; if the horse is unsteady, no-one should put themselves at risk.
Hose or pour cool water on the hindquarters, chest and inside of the hind limbs (where large blood vessels are located near the skin surface). If the horse will tolerate it and it is needed, consider also the head and poll (which houses the nerves responsible for sensing thermoregulation).
Opinions vary on whether to remove the heated water with a sweat scraper before more is added, so this comes down to personal preference and operator safety. Hosing alone can drop the horse’s body temperature about 2°C within 10 minutes.
Recent Japanese research, performed prior to the Tokyo Olympics, compared various techniques for cooling down overheated horses and corroborated this approach. In the study, both the rectal temperature and the temperature in one of the major arteries in the body (pulmonary) were recorded while the overheated horses received their cooling. The rate in fall of temperature, and the total fall in temperature were then compared.
There were five methods trialled: walking with no additional cooling; walking while exposed to fans; walking with the intermittent application of cold water which was scraped off; walking with the intermittent application of cold water which was not scraped off; and finally stationary with continuous application of water via shower hoses.
While all the cooling techniques made a positive difference, the stationary with continuous tap water was found to be the most effective, followed by the intermittent cold water with scraping and then intermittent cold water without scraping.
Offer cool water to drink, both plain and with a good commercial electrolyte added. Consuming any quantity of cool water will help decrease core temperature.
On arrival, the vet may administer an intravenous anti-inflammatory injection. They may also opt to sedate the horse to minimise violent movements. Cooling through dosing of fluids (either by stomach tube or intravenously) may also be performed.
It may take an hour or more to get vital signs back to normal. After a serious episode, the horse should have 10 to 14 days of rest with some turnout and a gradual return to work.
Prevention over cure
Although most horses who suffer overheating make a full recovery, prevention is always best. This is especially the case in horses who have seriously overheated before, as they tend to be more susceptible to overheating in the future.
The following measures will help prevent overheating:
Provide water. Maintain hydration by always allowing free access to water, especially on hot days. If your horse won’t drink, offer some hay which can stimulate drinking. Soup-consistency bran or pellet mashes are another means of getting extra water into him. Consider adding electrolytes, particularly if your horse has been sweating excessively.
Provide as much shade as possible. Limit what you do with your horse during peak heat if possible. Ride your horse in the early mornings, shorten your ride or go slower, and provide frequent breaks in the shade.
Ventilation is key when transporting. Provide open windows in trailers, but don’t let your horse stick its head out while on the road! Have a source of water while travelling, and don’t leave your horse in a parked trailer, especially if there is no shade. Just as with a parked car, temperatures inside a trailer can rapidly reach 60°C, with a horse quickly overheating in such conditions.
Stop exercising if your horse is showing early signs of overheating. Consider using fans on hot days. If your horse lives in a stable block with limited ventilation, try to arrange more air circulation by carefully placing a fan in front of the stable or in the passageway.
Acclimatisation. If moving your horse to a hotter climate to compete, you should arrive well in advance of the competition; a horse takes around two weeks to acclimatise to a new climate.